Terminal differentiation of chick muscle, in terms of the cellular mechanisms underlying the synthesis of the myosin heavy chain protein will be studied. We will isolate the myosin genes from a library of recombinant DNA, consisting of 20 kilobase pair fragments of chick DNA that have been cloned into bacteriophage lambda. (32P) complementary DNA, synthesized from a purified myosin mRNA template will be used as a probe for the myosin genomic sequences. The plaques that hybridize will be purified, and the presence of the myosin DNA sequences confirmed by a modified Hybrid-Arrested-RNA Translation Assay. The multiple myosin genes will be differentiated by restriction endonuclease mapping. The organization of the structural and non-structural myosin gene sequences will be determined by transfer of the restriction fragments to nitrocellulose, followed by hybridization of the material with 125I-myosin mRNA. The clone-derived myosin DNA will be used as a probe to map any structural differences that are present in myosin mRNA derived from two cellular compartments: 1. The heavy polysomes which are synthesizing protein. 2. The messenger ribonucleoprotein particles, or mRNP's which are inactive in protein synthesis. Differences will be determined by visualization of the (mRNP) mRNA-myosin DNA and (polys.)mRNA-myosin DNA R loops in the electron microscope, and by hybridization of 125I myosin mRNA with myosin DNA restriction fragments. The data will indicate whether the processing of translationally inactive myosin mRNA (mRNP mRNA) to active mRNA (polys. mRNA) is accompanied by sequence removal. Clone-derived myosin DNA will also be used to determine the myosin mRNA pools during myogenesis in primary chick cultures. The data will be correlated with the rate of myosin synthesis and the translational utilization of the myosin mRNA during development will be determined.